Ethanol as a fuel offers a host of potential benefits, according to its supporters. It can be grown and refined primarily in the U.S., whether made from corn, switchgrass or cellulose. It is already being used as a fuel additive—to help gasoline burn more completely and, thus, cut down on air pollution. And, because it is made from plants that pull carbon dioxide from the air, it does not add additional greenhouse gases to the atmosphere, which are driving climate change. But a new study shows that it will not help clear the nation's skies of smog; on the contrary, it could increase the levels of that dangerous pollution.

Environmental engineer Mark Jacobson of Stanford University used a computer model to assess how the air pollution in the U.S. would react if vehicles remained primarily fueled by gasoline in 2020 or if the fleet transferred to a fuel that was a blend of 85 percent ethanol and 15 percent gasoline, so-called E85. Under the latter scenario, levels of the cancer-causing agents benzene and butadiene dropped, whereas those of formaldehyde and acetaldehyde rose: In other words, it was a wash.

Because burning ethanol can potentially add more smog-forming pollution to the atmosphere, however, it can also exacerbate the ill effects of such air pollution. According to Jacobson, burning ethanol adds 22 percent more hydrocarbons to the atmosphere than does burning gasoline and this would lead to a nearly two parts per billion increase in tropospheric ozone. This surface ozone, which has been linked to inflamed lungs, impaired immune systems and heart disease by prior research, would in turn lead to a 4 percent increase in the number of ground level ozone-related deaths, or roughly 200 extra deaths a year. "Due to its ozone effects, future E85 may be a greater overall public health risk than gasoline," Jacobson writes in the study published in Environmental Science & Technology. "It can be concluded with confidence only that E85 is unlikely to improve air quality over future gasoline vehicles."

But estimates of the nitrogen oxides and unburned hydrocarbons released by ethanol combustion vary, according to Tim Gerlach, vice president of clean fuels and vehicle technologies at the American Lung Association of the Upper Midwest in St. Paul, Minn. "We ran a couple of vehicles in multiple dynometer runs and measured tailpipe emissions," he says. "[E85] compared very favorably to a low-sulfur, low-benzene, oxygenated gasoline." Specifically, burning E85 resulted in fewer ozone-forming compounds than gasoline. And E85's benefits as far as combating global warming outweigh any impact in ozone pollution. "We need to have an orderly, sustained implementation of low-carbon fuels and a smooth transition to a low-carbon world," says Roland Hwang, vehicles policy director at the Natural Resources Defense Council, an environmental group. "E85 is a part of the solution to global warming."

As a result of his ethanol finding, Stanford's Jacobson suggests that electric vehicles, such as hybrids that plug into the existing electrical grid or hydrogen fuel cell cars might prove a better solution to future vehicle energy needs from an environmental perspective. "We haven't reviewed the study here," says Julie Ruggiero, a spokesperson for the U.S. Department of Energy, which is currently pursuing ethanol research as part of President Bush's plans to increase its use as an alternative fuel. "Ethanol is just one part of a broader renewable portfolio."

Ethanol advocates agree. "Ethanol is not the silver bullet," says Matt Hartwig, a spokesperson for the Renewable Fuels Association, an industry group. "It's a very important tool in the toolbox to address energy security and to address the issues around global warming and the environment. But it's not the only answer." And Jacobson's study may have revealed one of the downsides to this alternative fuel.

Scientific American is part of Springer Nature, which owns or has commercial relations with thousands of scientific publications (many of them can be found at www.springernature.com/us). Scientific American maintains a strict policy of editorial independence in reporting developments in science to our readers.